1. Field of the Invention
The present invention relates to a recording medium, particularly, to a holographic optical recording medium.
2. Description of the Related Art
The development of an information storage device and an information storage method is being directed to the increase in the recording capacity. As a part of the development, proposed is a holography system using a holography display. As a recording medium of such a recording system, proposed is a so-called “photopolymer” utilizing a change in the optical characteristics caused by, the polymerization of a photo-polymerizable compound.
In the recording medium containing a photopolymer, shrinkage of the recording medium is induced by the polymerization of the photosensitive monomer. To be more specific, since the polymerized monomer occupies volume smaller than that occupied by the individual monomer, a local shrinkage is generated in each step of writing data in the recording medium. Where locally shrunk regions are combined, it is difficult to withdraw accurately the stored data.
As a measure for the volume compensation in a holographic optical recording medium, proposed in the past was a combination of systems capable of forming an optical image and capable of phase separation from the polymer matrix or a combination of compounds that can be cleaved by an acid.
However, in any of these measures, the recording layer of a recording medium is formed under the state that compounds having a low molecular weight are dispersed in a matrix. Where migration of the low molecular weight compounds within the recording layer is expected in the composition, it was difficult in the past to guarantee the stability during the store of the recording medium. Also, since the composition of the recording layer material containing a low molecular weight compound is in a liquid form, required is a liquid holding mechanism in forming a film. To be more specific, required is a bank structure for retaining a liquid film in a space between adjacent holding substrates. Further, in a solution of the composition of the recording layer material, a change in the composition is generated by the evaporation of the component, with the result that the film quality tends to be rendered nonuniform by, the phase separation of the component after the film formation.